The present invention relates to cellular communications, and, more particularly, is directed to arrangement of antennas in a cell.
FIG. 1 shows cell 10 of a conventional cellular communication network having a transmitting antenna 20. In fact, the coverage area of antenna 20 has an approximately circular shape, but it is convenient to model the cellular network as formed of hexagonal (rather than circular) cells. Antenna 20 is modelled as being at the center of cell 10, but practically, it may not be precisely centered with respect to the location of other antennas.
As shown in FIG. 2, antenna 20 transmits message A at a carrier frequency f.sub.1. When the signal to be transmitted comprises time division multiplexed signals A, B, C, the transmission from antenna 20 is as shown in FIG. 3.
FIG. 4 shows receiving apparatus 50, such as a cellular telephone. Antenna 51 receives messages transmitted from antenna 20, and possibly from other antennas in other cells, supplies a received signal to receiver circuit 52, which processes the received signal to extract the message transmitted by antenna 20. Receiver circuit 52 supplies the extracted message to processor 53, which converts the extracted message into a user perceivable signal, such as an audible speech signal, and applies the user perceivable signal to amplifier 54 which is adapted to convert the user perceivable signal into an acoustic signal.
Receiving apparatus 50 may be a pager, in which case the user perceivable signal is a visually displayable signal, and instead of amplifier 54, a display is provided. Alternatively, receiving apparatus 50 may also have transmission capability (not shown).
To increase the traffic carrying capacity of a cell, various schemes are used.
FIG. 5 shows cell 11 having antennas 21, 22, 23 located therein, and transmitting at carrier frequencies f.sub.1, f.sub.2, f.sub.3, respectively. As shown in FIG. 6, each antenna transmits a separate message, indicated as A, B, C. Of course, the signal at each carrier frequency may use a time division multiple access (TDMA) scheme. For example, f.sub.1, may represent messages D, E, F, D, E, F, . . . , f.sub.2 may represent messages G, H, G, H, . . . , and f.sub.3 may represent messages I, J, K, L, I, J, K, L . . . The antennas 21, 22, 23 may be a single antenna.
FIG. 7 shows cell 12 having antennas 24, 25, 26 located therein, and each transmitting in a wide bandwidth channel centered at carrier frequency f.sub.1. However, as shown in FIG. 8, each of the antennas 24-26 uses a distinct modulating code, so that a receiver demodulating its received signal with the same distinct code properly recovers the transmitted message from the intended one of antennas 24-26. The scheme illustrated in FIGS. 7 and 8 is referred to as a spread spectrum scheme, in particular, a code (or carrier) division multiple access (CDMA) scheme. Here, too, the signal transmitted by each of antennas 24-26 may be a time division multiplexed signal.
Edge excited cells have been proposed in which three antennas are located at respective vertices of a hexagonal cell. In known CDMA systems, each of the antennas transmits the same signal. A transmission from a mobile station is received correctly if at least one of the antennas has correctly received the transmission. This configuration of antennas in a cell is also referred to as a corner-fed hexagonal pattern.
Since traffic continues to increase, additional techniques for increasing the traffic carrying capacity of a cell are sought.